Polymeric analytes can be detected using current methods such as chromatograph, electrophoresis, binding assays, spectrophotometry, etc. For example with nucleic acids, DNA concentration has been detected spectrometrically by measuring absorbance of a sample at 260 nm. This method gives reliable detection and quantification at relatively high concentration of DNA; however, it suffers from poor sensitivity at low concentrations.
Other methods for DNA detection include binding DNA to a fluorescence dye and detecting the fluorescence using a fluorometer. Examples of such a dye are PicoGreen®, which is commercially available through Invitrogen (Carlsbad, Calif.), and dyes disclosed in U.S. Pat. Nos. 6,664,047, 5,582,977, and 5,321,130. Although highly sensitive, the fluorometer methods are generally cumbersome, requiring reagent preparation and handling and special fluorometer for exciting and measuring fluoro-emission.
Likewise, proteins can be detected spectrophotometrically by measuring absorbance at 280 nm. Another method for protein detection is the Lowry assay. This method is based on the reduction phosphomolybdic-tungstic mixed acid chromogen in the Folin-Ciocalteu's phenol reagent by protein resulting in an absorbance maximum at 750 nm. Other methods to detect protein include the Bradford assay and the Biuret assay. These assays suffer from either low sensitivity or intensive laboratory preparation and procedure.
The presence of carbohydrates can also be determined spectrophotometrically by detecting their reducing ends using dinitrosalicylic acid. This involves heating a sample in the presence of dinitrosalicylic acid in boiling water the measuring the absorbance at 540 nm. Again, this method suffers from low sensitivity and intensive laboratory preparation and procedure.
Generally, detection of analytes in a microfluidic apparatus requires 1) high sensitivity because the analyte concentration is generally low; and 2) that the method be simple, requiring minimum number of reagents and fluid handling. Therefore, especially in the microfluidic arena, there remains a need for a simple, fast, and sensitive method and apparatus for detecting nucleic acids.